This invention relates to load pull testing of radio frequency transistors and amplifiers, at frequencies typically between 10 MHz and 150 MHz, under high power operating conditions. The test method uses automatic impedance tuners for synthesizing user defined impedances at the input and output of the devices under test (DUT) at the fundamental and up to two harmonic frequencies.
Load Pull is a measurement technique, in which the source and/or load impedance presented to a DUT (typically a power RF transistor) is changed systematically using pre-calibrated impedance tuners, while measuring the DC and/or RF performance of the DUT. This technique is popular at frequencies in the GHz frequency range but, hitherto, unknown in the frequency range below 100 MHz.
Accurate design of high power amplifiers, oscillators and other active components used in various communication systems requires accurate knowledge of the active device's (RF transistor's) characteristics under high power operation conditions. In such circuits, it is insufficient and inaccurate to describe transistors, operating in compression and at extreme high power in their highly non-linear regions, close to saturation (“saturation” means an increase in input power does not result into any increase in output power, “compression” means that the output power increases less than proportionally to the input power), using analytical or numerical models only. Instead the transistors need to be characterized using specialized test setups under the actual operating conditions.
The popular “load pull” and “source pull” method for testing and characterizing such components (transistors) under high power operation conditions has been developed at much higher frequencies in the GHz frequency range. Load pull or source pull are measurement techniques employing RF impedance tuners and other RF test equipment, such as RF signal sources and RF power meters. Since transistors are typically used close to power saturation conditions in high efficiency amplifiers, their internal nonlinearities deform the input signal and this deformation is revealed as a number of harmonic frequency components in the output signal. Those frequency components, if not terminated with the appropriate RF harmonic impedance, will degrade the performance of the amplifier. In order to generate the proper harmonic RF impedance a Harmonic Load Pull system is required. In such a system certain frequency selective components, such as frequency discriminators (Diplexers for 2 frequencies, Fo and 2Fo, or Triplexers for 3 frequencies Fo, 2Fo and 3Fo), see ref. 9 and 10, are used, whose task is to guide the harmonic components generated by the DUT into different paths, where they can be treated separately (FIG. 1); wideband impedance tuners can be used, in this case, since the frequency separation is done before the signal arrives at the tuners. Harmonic impedance tuners (see ref. 6 and 7) are also used in order to manipulate the harmonic microwave impedances under which the Device under Test (DUT, amplifier or transistor) is tested (FIG. 2). Note: Commercial Triplexers in this frequency (see ref. 10) are not frequency adjustable and are fixed designs for specific telecom frequency bands, not for harmonic frequencies. For instance, the Triplexer in ref. 10 has a main channel from 9.8 to 10.2 MHz, and secondary channels at 852-1872 and 3300-4620 MHz respectively, unrelated to harmonic frequencies. For harmonic load pull one requires “harmonic Triplexers”, not available at these MHz range frequencies.
FIG. 1 shows a setup using a frequency diplexer or triplexer allowing creating three independent frequency paths, loaded each with a wideband tuner. This concept of harmonic tuning is well known in the literature and is valid for all frequencies for which appropriate components, such as tuners and diplexers/triplexers, exist. Because of lack of such components harmonic load pull test systems are not known for frequencies below 800 MHz. This, on the other hand is due to the difficulty in manufacturing such wideband components in coaxial or waveguide because of their required size to handle low frequencies and large wavelengths. The MHz frequency range is for L-C components.
A simpler solution for a GHz-range harmonic load pull test system will use a single multi-harmonic tuner (see ref. 7), i.e. a tuner which allows independent tuning at harmonic frequencies without using frequency discriminators, such as diplexers and triplexers. Such a setup is shown in FIG. 2.